We now have now seen two circuits that can
be built with the NE555, one with no steady states (the astable), and
one with one steady state (the monostable).
The third circuit, shown in figure 1 has two steady states and is
called a bistable.
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555 Timer Datasheet 141Kb |
The type of bistable that can be made with the NE555 is one with two inputs. A low pulse on the Trigger input (pin 2) makes the output go high; a low pulse on the Reset input (pin 4) makes the output go low. There is no timing involved in this circuit, so therefore there are no equations to work out the components. Notice in this circuit that pin 4, the Reset pin is used. In both the astable and monostable circuits this was connected to +Vs where it has no effect. Connecting it to 0V resets the NE555 so that its output goes low. Therefore, if Reset is used in the monostable circuit, the timing is stopped and the output returns to 0V. In the astable circuit, Reset will force the output to 0V. When Reset returns to +Vs, the astable timing will resume.
Applications of the bistable circuit in
Meccano could be an automatic reversing switch in a model such as
a locomotive. The output would need to be connected to a relay to
drive a motor, the relay being wired as a reversing switch (see
page 2 for an example). When the locomotive reaches one end of
the track, a switch makes the Trigger input to the NE555 go low,
causing the output to change state and the locomotive to change
direction. A similar situation will occur at the other end of the
track, except another switch will make the Reset input go low.
See also: The NE555
astable circuit for an
explanation of the NE555 timer IC
Contrast with: The NE555
astable circuit , The NE555
monostable circuit
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Buy 555 Bistable Modules from the Electronics in Meccano Circuits Shop |
Simple
motor control
By Stan Leech
Reversing a DC motor using an AC supply
This simple circuit, shown in figure 3, requires only two 1A diodes and an SPDT toggle or slide switch with centre-off. The AC supply from a transformer needs to be 1.5 to 2 times the voltage required by the DC motor that you use. The reason for this is that the diodes, whilst converting from AC to DC, lose half of the power in the process.
Momentary action switches
Mention was made in issue 1 of difficulty in obtaining momentary action switches. They are also known as 'biased switches' and return to the centre off position when pressure is released from the toggle.
They are useful for controlling motors in exhibition models when the attention of an exhibitor is distracted by a person asking questions. A mechanism reaches the end of its traverse, and failure to stop or reverse results in either damage to the motor or a jammed up mechanism.
These switches can be mounted in a small black box (75mm x 50mm x 25mm) as shown in figure 4. Two terminal blocks form the power supply input at 6V or 12V, and six terminal blocks provide connections for three motors. These are mounted at each side of the case. This set up is ideal for, say, a block setting crane with three motors.
Squires in Bognor Regis can provide the parts required to build these circuits. Although Squires prices are higher than Maplins', a minimum order of £7.50 is post free and their 332 page catalogue is also free. Their address is:
Squires Model and Crafts,
100 London Road,
Bognor Regis,
West Sussex PO21 1DDTel: 01243 842 424
Fax: 01243 842 525
See
also: Rotary switch motor controllers , Motor
control with a Darlington Pair
In electronic components catalogues you may sometimes see a number followed by the letters VA. This is simply another way of indicating the power rating, in Watts, of a component or appliance (for DC and purely resistive AC loads only) and it arises from an equation called the Power Law...
...where P is power in Watts, I is current in Amps and V is voltage in Volts. VA is used simply because Volts times Amps gives Watts, so, for example 50VA is the same as 50W.
The Power Law can be re-arranged to find out the maximum current required by a motor or appliance given the power and voltage. So, for mains voltage motors or appliances, which normally specify their power rating somewhere on the casing, you can work out which fuse you need for the plug.
The example {in curly brackets} shows a typical iron consuming 1000W, which therefore has a current of 5.6A flowing through it. Choose the minimum value fuse which can pass the appliances normal current and ensure that the mains flex can pass the fuse current (not the appliance current which will be lower.) If the flex cannot carry the fuse current it may overheat and burn out before the fuse blows, possibly causing a fire. Note that it is the fuse (not the appliance) that determines the flex rating, so the iron in the example above would require a 13A fuse (the smallest standard value which can pass 5.6A), so the flex must be capable of safely passing 13A.
The Power Law can also be used to work out the fuse required for low voltage circuits, and is a handy equation to remember in electronics. Note though that components such motor and large capacitors take a large surge or current when first switched on, which may blow fuses that are only just rated high enough, so circuits with these components in them may require fuses with higher ratings.
Using mains power in models
Mains power should not be used to power mains voltage motors in Meccano models, even if the model chassis has been earthed. Earthing is not enough to ensure the safety of a model. Even though the earthing on a model may function correctly, the power point used to power the model may have a faulty earth connection. This means that if a fault develops in the model and the model chassis becomes live, the earth wire will become live also. If someone touches the model they would therefore receive an electric shock. To avoid the possibility of this situation occurring, it is clearly best not to use mains power directly in Meccano models unless an isolating transformer is used.
Continued from Building a
power supply in issue 4
The previous article about a simple power supply in issue 4 of Electronics in Meccano explained the various stages required to produce regulated DC from a 230V AC mains supply. This article deals with some of the more complex matters which you may need to consider when building a simple power supply.
Rectification loss
Firstly, the diodes used to rectify the low voltage AC supply to produce DC are not perfect. The output voltage from a diode will be about 0.7V less than the input voltage to that diode. This means that in the full-wave rectifier, where two of the diodes are in use at any one time, there will be a 1.4V voltage drop. So, if you put 12V in, you will get 10.6V out.
Smoothing
The next stage after rectification is smoothing, which is provided by a large value capacitor. The output from the rectifier has an average voltage somewhere between 0V and Vs since it is still alternating between these two voltages. The smoothing stage produces a voltage near to Vs, but with a slight ripple voltage. This means that there appears to be a larger voltage after smoothing than before it.
Ripple voltage
The ripple voltage present after smoothing should be as small as possible since it can cause any ICs (especially logic ICs, but not normally the NE555) in the circuit that you are connecting to the power supply to misbehave. If you find that your circuit is not doing what it should be, try using a smoothing capacitor with a larger value to see if that solves the problem. You can work out the ripple voltage if you need to using the formula
...where Vr is the ripple voltage in Volts, I is the current consumed by your circuit in Amps, C is the value of the smoothing capacitor in Farads, and f is the frequency of the AC supply in Hertz (which will be 50Hz in the case of the mains.) The ripple voltage Vr should not be more than 10% of Vs.
Regulation
If you are using a regulator IC as the stage after smoothing, then you shouldnt need to worry about the ripple voltage, because the whole point of having a regulator is to generate a stable, accurate, known voltage for your circuit! However, if the ripple voltage is too large and the input voltage to the regulator falls below the regulated output voltage of the regulator, then obviously the regulator will not be able to produce a correct regulated voltage. The input voltage to a regulator should normally be at least 2V above the regulated output voltage of the regulator.
In this issue Practical Matters takes a look a some of the different connectors available for multiple wire connections.
Pin strips
Power connectors
Audio connectors
D-type connectors
DIN connectors
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The
following lists the electrical parts that are discussed in the
articles. Prices and order codes given are taken from the current
Maplin catalogue, which is the probably best source of electronic
components for the hobbyist in the UK.
If you have access to a company account with Rapid Electronics or RS Electronics you may find these companies are cheaper.
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Maplin charge £2.50 for delivery on orders under
£30.00 inc. VAT.
Prices are taken from the September 2000 - August 2001 Maplin catalogue, and
include VAT at 17.5%
Contact their order line on 0870 264 6000 or visit one of their shops.
Their customer service line is 0870 264 6002 and
they have a website at www.maplin.co.uk where on-line ordering is
available.
www.eleinmec.freeserve.co.uk |
Electronics in Meccano September 1999 -- Issue 5 Edited by
Tim Surtell |
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